Electron accelerator provided with starting auxiliary



1950 R. v. LANGMUIR ETAL 2,528,525

ELECTRON ACCELERATOR PROVIDED WITH STARTING AUXILIARY Filed May 22, 19473 Sheets-Sheet 1 Invent, ors: Roberbvbangmuir, Herbert, C. Pol iock,Willem FWestendorp,

The-i T Attorney.

9 1950 R. v. 'LANGMUIR ETA 2,528,525

ELECTRON ACCELERATOR PROVIDED WITH STARTING AUXILIARY Filed May 22, 19473 SheeTs-Sheet 2 Fig. F724.

l\ k k z R 0 g 8 CAPACITOR CURRENT g 5 v) a) 3 X3 8 g E MAGNET CURRENT R5 3/ q R I 2 g \1 VOLTAGE VOLTAGE 5 K) T \I Fig. 5.

\\ E g E CURRENT THRU VAR/ABLE nvuucrmvcz Inve nt, 01" s 'Roberbvbangmuir,- Herbert, CPO! lock Willem FWestendovp,

Their Attorney Nov. 7, 1950 R. v. LANGMUIR E'TAL ELECTRON ACCELERATORPROVIDED WITH STARTING AUXILIARY 3 Sheets-Sheet 3 Filed May 22, 1947Fig.6.

Inventors: Robert-\fipanggm fi,

Herbert QPollock,

Willem FTWestendor Their Attorney.

Patented Nov. 7, 1950 ELECTRON ACCELERATOR PROVIDED WITH STARTINGAUXILIARY Robert V. Langmuir, Herbert G. Pollock, and

Willem F. Westendorp, Schenectady, N. Y., assignors to General ElectricCompany, a corporation of New York Application May 22, 1947, Serial No.749,852

6 Claims. 1

The present invention relates to apparatus for accelerating chargedparticles, and it is particularly directed to the provision of means forcontrolling the magnetic fiux in such apparatus.

An apparatus in which charged particles are accelerated by atime-varying magnetic flux is commonly known as a Betatron. Theoperation of betatron apparatus is described in Journal of AppliedPhysics, volume 16 (1945). An apparatus whereby charged particles areaccelerated successively by an electric field is commonly known as aSynchrotron. Its operation is described by E. M. McMillan in thePhysical Review, volume 68, page 143.

A prior application, Serial No. 639,462, filed January 5, 1946 byHerbert G. Pollock and Willem F. Westendorp, which is assigned to thesame assignee as the present application and has now issued as PatentNo. 2,485,409, describes apparatus whereby charged particles are causedto be accelerated in an orbital path by cyclically varying magnetic andelectric fields operating in succession. The initial acceleration of thecharged particles is caused by a time-varying magnetic flux. When thecharged particles have been accelerated to a predetermined velocity bythe magnetic flux, they are additionally accelerated by a' cyclicelectric field which acts upon the gyrating particles at selectedportions of their orbital path.

The present invention is applicable both to apparatus in which theacceleration of charged particles may be carried out wholly by amagnetic field, as described, for example, in Westendorp U. S. Patent2,394,071, patented February 5, 1946 and to apparatus such as isdescribed in the above-mentioned Pollock and Westendorp application inwhich magnetic and electric fields successively function to causeacceleration.

In a betatron such as described in the aforementioned Westendorp patent,the magnetic accelerating flux is generated in a magnetic core which isprovided with one or more windings, arranged to be energized byalternating current. In circuit with the winding or windings is providedsufiicient electric capacity to result in resonance at the operatingfrequency of the apparatus. During normal operation of a betatron, theexcitation of the resonant electron-driving circuit occurs with butlittle power required to be supplied from an external source. In aproperly designed system only about one per cent, or even less, of thecirculating power in such resonant circuit represents a loss which mustbe compensated for by a supply of power.

In an accelerator in which acceleration of parfield, ordinarily littledifiiculty is experiencedin starting such apparatus, as all circuitelements are linear. I I

However, apparatus providing for acceleration of electrons by asynchronous electric field subsequent to their acceleration by a varyingmagnetic flux is provided with a smaller central core which saturatesearly in the cycle of the magnetic field and remains saturated until themagnetic field returns to a value near zero /120 of a second later forcycle operation. When the accelerator is started the inductance andcapacity of circuit containing the magnet windings are so far fromresonance that excessive current would be drawn from the power lines ifit were attempted to start the apparatus at normal operating frequency.No voltage will build up unless line currents many times greater thanneeded for full voltage steady state operation can be furnished.

In accordance with our present invention this difficulty is overcome byproviding separate magnetic means for suitably regulating the effectivetuning inductance of the electron-driving magnet as may be required toproduce a condition'of resonance with the associated capacitors atstarting. In accordance with one of the embodiments of our invention asupplemental winding is pro-. vided on the central core of anaccelerator. In circuit with this winding is provided a variableinductance .device whereby the magnetic flux may be varied and desiredresonance can be produced at will by suitable regulation with minimumline current going into the system.

In accordance with another feature of our invention, the regulableinductance device in circuit with a supplemental winding on the core ofthe accelerator is utilized during normal operation of an accelerator(i. e. subsequent to starting) for determining the orbit radius of theaccelerated charged particles. It is desirable in a combinationbetatron-synchrotron device, such as described in the abovementionedPollock and Westendorp application Ser. No. 639,462, that the orbitdiameter at the close of betatron operation should match within about 1the orbit diameter required for synchrotron operation. If a substantialdifierence should exist the electrons will be out of phase with theradio frequency accelcrating electric field. The means provided by ourinvention of tuning the magnetic flux of the ,magnet core of theaccelerator is operable for controlling the radius of the orbit of thegyrating particles while the accelerator is in operation.

Hence, it provides a means for adjusting theacceleration orbit formaximum power output.

These and other features of our invention will be described more fullyin connection with the accompanying drawings, in which Fig. l isvertical section of an accelerator which is provided with an alternatingcurrent flux regulator; Fig.

2 is a top View of the accelerator shown partly in section; Figs. 3 and4 are graphs of currenta voltage relations; Fig. 5 is a diagramillustrating the relation of the orbit radius to the regulator current;Fig. 6 is a vertical section of an accelerator which is provided with aflux regulator operated by direct current excitation and Fig. 7 is adiagram of an operating circuit for an accelerator which is providedwith a direct current flux regulator.

In Figs. 1 and 2 is shown a sealed, rotationally symmetric container ID,which provides an annular path for the acceleration of the chargedparticles. Ordinarily it consists of glass. Eleotrons are provided by anelectron gun l l, the energizing conductors l2 of which are sealed intoa side arm l3. As our present invention is not concerned with theconstruction of the electron gun and as the electron gun is described inprior patents and other publications, it will not be herein described.The container It is highly evacuated and as shown in Fig. 2, is providedon its interior surface with coatings I4, is of silver or other suitableconductive material which are separated by gaps 16, H. The function ofthe electrodes which are longitudinally subdivided adjacent the gaps, asindicated, will be described here inafter.

The acceleration chamber i0 is supported between the pole pieces I8, 19of a rectangular magnetic frame 20 having a centrally located magneticmember 2i, traversing'the axis of the annular chamber l0, and beingspaced frornthe main core structure by insulating spacers 22, 22. Thecore member 2| as shown in Fig. 2, has a radially laminated structure.The core members I8, I9 consist also of radially placed laminations ofvarious radial dimensions. The windings 23, 24 mounted on the polepieces l8, 15 are connected in series by the conductors 25. The windings23, 24 receive energy as secondaries from the primary windings 26, 2?which are energized froma source of alternating current (not shown).Capacitors of suitable size and number, symbolically indicated at 28,are provided in the circuit 25 to resonate at a frequency correspondingto the desired operatingfrequency of the apparatus, for example, at 60cycles per second.

When the apparatus is started, and before the core member 2! ismagnetically saturated, the windings 23, 24 and the capacitors 28resonate at a frequency differing from the normal operating frequency.The apparatus hence would draw an excessive starting current from thesupply source as illustrated by the graphs of Figs. 3 and 4. Thewattless component of the capacitor current, graph 29, would vary withthe voltage at a difierent rate than the wattless component of theinductive part of the load, graph 3&3. The graph 30 shows the wattlesscomponent at different voltages of the combined wattless ampere turns ofcoils 2B, 23, 24 and 2'! divided by the number of turns of coils 23 plus24. The resultant wattless current in the circuit of the windings 26, 2]which would be obtained at diiferent voltages is indicated by the group3!- of Fig. 4. The preponderance of capacity in the system 1m} 4 tiallywould cause a large leading wattless current to flow. As the coresaturates with rising applied voltage, the wattless component wouldbecome lagging as indicated by the graph 3! passing through Zero. Theactual power loss current, or watt component, is too small to beindicated on the scale of Fig. 4.

The difliculty described is avoided in accordance with our invention byproviding a winding 32 on the core member 25 and connecting this windingby the conductors 33, to a variable inductance device, which isconventionally indicated at 35. When the circuit is closed by the switch3%, the magnetic flux in the core member may be varied by suitableregulation of the inductance device 35 to establish and maintain adesired state of resonance in the circuit 25 until the accelerator isfully energized. The switch 36 then may be opened.

During normal operation of the accelerator the electron beam preferablyshould be in thecenter of the evacuated acceleration chamber asindicated by the dot in the center of the chamber ill, Fig. 1. Thefulfillment of this condition depends on the relation between the fluxdensity at the orbit and the flux through the orbit during the inductionacceleration or betatron phase of the acceleration cycle, as expressedby' the formula By changing the flux'oo in this relationship,

the radius R0 assumed by the electrons (orbit radius) can be adjusted atwill. The flux can be varied by means of the current in the circuit ofthe coil 32. adjusting the inductance 35 An adjustable resistance inthis circuit would n'otresult in the proper variation of current withtime;

Fig. 5 shows the relationship between orbit radius and the currentthrough the variable inductance. If the orbit is to be maintained at avalue R1 the current should be adjusted'to a value I1. Since the currentflowing through the coil 32 and the inductance suppresses the flux inthe central core member 2!, the radius will decrease as the current inthe winding 32 increases as shown in Fig. 5.

The shift from magnetic to synchronous high frequency field accelerationmay be made automatic as will be described in connection with Fig. 7. Itshows asystem whereby the effective inductance of the magnetizingcircuit is ener-.

gized by unidirectional current controlled by'a regulator. The lattersystem is described and claimed in a co-pending application, Serial No.749,853, filed concurrently herewith'by Willem F. Westendorp. V As shownin Fig. 6 the magnetic core 20 inthis modification is partially excitedby direct current windings 31, 3B which are located on the outermostlegs 39, 40 of the magnetic core 20. .The coils 31, 38 are connected toa source of direct current in series with a regulating resistance asshown in Fig. 7. Alternating current voltages which are induced in themneutralize one another and do not appear across the direct currentsupply. By an automatic device, which will be described in connectionwith Fig. 7, the direct current excitation is reduced as the excitationof the magnetization of the core 20 builds up until frnally when thealternating current excitation is normal, no direct current remains in.the wind;- ings 37, 38. r

th d g pr lectrical connection; as? 7,

This current can be varied by alternating current excitation is suppliedby a source (not shown) through an input circuit 4|, 42 in series with avoltage regulator 43 to the primary windings 26, 21 which areinductively related to the secondary windings 23, 24. The latter areconnected in series with one another and in series with'the capacitors28. The windings 31, 38 as previously stated, are excited by directcurrent which here isshown as being supplied by the conductors 49, 50.The latter are connected to a" 'source' of direct current, representedby the supply conductors, 51, 58 and 59, in series with a regulableohmic resistance and a second series resistance 52. Although capable ofbeing adjustable, as shown, the resistance 52 ordinarily is not variedduring normal operation. Theregulable resistor switch blade 53 is gearedto the shaft 54 of the rotor 55 of a direct current motor. The fieldwinding 56 of this motor is connected to the conductor 59 and to thegrounded conductor =58 of a three-wire direct current supply system.Conductor 59 of this supply system is connected by a conductor 60, inseries with a limit switch 6| to a stationary contact 62. A secondstationary contact 63 is connected by a conductor 64 in series with alimit switch 65 to the direct current conductor 51.

When the apparatus is deenergized, a movable switch blade 66 is causedby the weight 61 to make contact with contact 53, causing the motor 55to move the blade 66 to the right, thereby reducing the resistance ofthe circuit. When the apparatus is started maximum exciting directcurrent thus is supplied to the windings 31, 38. By the direct currentexcitation of windings 3T, 38 a desired condition of resonance isestablished in the circuit containing the windings '23, 24. As theexciting alternating voltage is increased by the voltage regulator 43,the excitation of the stationary member 12 of a Selsyn which isenergized by a transformer 68, the primary winding of which is connectedacross the circuit 4|, 42 is increased. The movable member 69 of theSelsyn is connected by slip rings to a current transformer 1| which isenergized from the circuit 4|, 42 as indicated. As excitation builds upin the magnet 20 and the circuit of the windings 23, 24 approximates astate of resonance the movable member of the Selsyn device first opensthe contact 63 and thereupon causes the switch arms 66 to close thecontact 62. This change in connections energizes the armature 5 5 torotate in a reverse direction and insert resistance 5| into the directcurrent excitation circuit 49, 50 of the windings 31, 38. The excitationof the-windings 3T, 38 thus is decreased and eventually these windingsare deenergized by the circuit 50 :being opened.

Although the present invention is not concerned with the starting andoperating circuits, such circuits are indicated conventionally in Fig.'7. As described in Westendorp U. S. Patent 2,394,071, the magneticpeaking strip 14 changes cyclically and abruptly from a saturated to anunsaturated condition in accordance with the variation of magnetic fieldand thereby controls the injection of electrons. When the switch 13 isclosed, the variation of current in the winding of the peaking strip 14through the intermediary of a control device energizes a pulse generator16 which feeds an electron gun (Figs. 1 and 6) whereby electrons areintroduced into the timevarying magnetic field generated by the windings2|, 22. As conventionally indicated in Fig. 7, the electron guncomprises a thermionic filament 11,

a focussing cup 78 and a housing 19. After a predetermined time intervalduring which the electrons are accelerated solely by a magnetic field, adelayed action switching device 80 connects a high frequency source 8|to the electrodes 4, l5 on the interior of the accelerator chamber, asdiagrammatically indicated in Fig. 7.

After a predetermined period of acceleration a second electronic delaydevice 82 effects a discharge of the accelerated electrons for exampleupon a target 83 (Fig. 1), as is Well understood.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

In an accelerator for charged particles which is provided with amagnetic core and magnetizing circuit therefor containing capacitors forimpressing a time-varying magnetic field on said particles theimprovement which consists in the combination of a winding surroundingsaid core, and only a variable inductance connected to said winding inclosed circuit therewith.

2. In apparatus for accelerating charged particles in an orbital path thcombination of a loop-shaped magnetic core having a magnetic memberextending across said loop and traversing the area enclosed by theorbital path, magnetizing windings for said core, a capacity circuitcoupled with said windings, an auxiliary winding on said transversemagnetic member, and a regulable inductance device external to saidaccelerator and in circuit with said auxiliary winding.

3. An electron accelerator comprising the combination of an annularevacuated container providing an orbit for the acceleration ofelectrons, means for introducing electrons into said orbit, a magneticcore having pole pieces respectively adjacent to and on opposite sidesof said container and also having a magnetic member traversing the axisof said container, said axial member being magnetically saturated duringnormal operation of said accelerator, magnetizing primary windings onsaid pole pieces, secondary windings inductively related thereto,electric capacitors connected to said secondary windings to provide acircuit, inductance and capacity proportioned to be resonant at apredetermined frequency under operating conditions but beingnon-resonant under starting conditions, a magnetic winding on said axialmember and a regulable inductance device connected to said winding andbeing located external to said accelerator whereby said circuit may betuned to permit said accelerator to be started.

4. In magnetic induction apparatus for accelcrating charged particles inan orbital path, said apparatus having a rotationally symmetricalmagnetic core and magnetizing windings for impressing upon saidparticles a time-varying magnetic flux which links said orbital path toaccelerate said particles and a time-varying magnetic field whichconstrains said particles to said orbital path, said magnetizingwindings having sufficient capacitance in circuit therewith to beresonant under operating conditions of voltage and frequency, theimprovement which comprises auxiliary rotationally symmetrical windingson said core energized by inductive coupling thereto and having incircuit therewith means for regulating the alternating current flowingin said latter windings whereby said time-varying flux linking saidorbital path may be altered.

5. In magnetic induction apparatus for accelerating charged particles inan orbital path, said apparatus having a rotationally symmetricalmagnetic core and magnetizing windings for im- 7' pressin upon saidparticles a time-varying magnetic flux which links said orbital path toaccelcrate said particles anda time-varying magnetic field whichconstrains said particles to said orbital pressing a time-varyingmagnetic field on said particles, the improvement which comprises thecombination of a winding surrounding said core and only a variableinductance and a switch conpath, said magnetizing windings havingsufficient 5 nected to said winding in series circuit therewith.

capacitance in circuit therewith to be resonant under operatingconditions of voltage and frequency, the improvement which comprisesadditional windings on said core energized by inductive couplingthereto, said additional windings 10 having electrically connectedthereto only a dissipative circuit capable of regulating the alternatingcurrent flowing in said additional windings whereby said time-varyingflux linking said orbital path may be altered.

6; In an accelerator for charged particles which isprovided with'amagnetic core and magnetizing circuit therefor containing capacitors forim- ROBERT V. LANGMUIR. HERBERT C. POLLOCK. WILLEM F. WES'IENDO-RP.

REFERENCES. CITED UNITED .STATES PATENTS 5 Number Name Date 2,297,305Kerst Sept. 29, 1942 2,331,788 Baldwin Oct. 12, 1943 2,394,071'Westendorp Feb. 5, 1946

